1. Field of the Invention
This invention relates to a pre-drive circuit for use in a switching regulator, a DC/DC converter, etc.
2. Description of the Related Art
A pre-drive circuit which performs the on-off control of a switching transistor element through a pulse transformer is one which on/off controls the power transistor of a switching circuit by the use of two drive transistors, as illustrated in FIG. 4.
With reference to FIG. 4, a switching circuit 1 on/off controls a power source PS for a load L by means of a power transistor Q.sub.1 and is connected to a control circuit 3 through a pre-drive circuit 2. Connected across the base and emitter of the power transistor Q.sub.1, through a parallel circuit consisting of a resistor R.sub.1 and a capacitor C, is a secondary coil S of a pulse transformer T. On the primary side of the pulse transformer T, a transistor Q.sub.2 is connected to a first primary coil P.sub.1, and a transistor Q.sub.3 is connected to a second primary coil P.sub.2.
In turning on the power transistor Q.sub.1, a signal output transistor Q.sub.5 of the control circuit 3 is turned off to turn on the transistor Q.sub.2. When the transistor Q.sub.2 is turned on, voltages in polarities indicated by solid lines in the figure are generated in the pulse transformer T, and current flows to the base of the power transistor Q.sub.1 to bring this power transistor Q.sub.1 to the on state.
In turning off the power transistor Q.sub.1, the signal output transistor Q.sub.5 is turned on which turns off the transistor Q.sub.2, while at the same time, a signal output transistor Q.sub.4 is turned off to turn on the transistor Q.sub.3. The base charge of the power transistor Q.sub.1 is extracted by voltages in polarities indicated by the broken lines in the FIGURE, thereby turning on this power transistor Q.sub.1.
With such a prior art pre-drive circuit, the transistor Q.sub.2 is turned off and the transistor Q.sub.3 is simultaneously turned on in order to turn off the power transistor Q.sub.1. In this regard, the conventional transistors involve a delay in terms of the response of collector current to base current, and when control signals SA and SB to be applied to the pre-drive circuit 2 are provided with exactly opposite phases, the transistors Q.sub.2 and Q.sub.3 are turned on simultaneously. At this moment, excess currents flow through both the transistors Q.sub.2 and Q.sub.3, resulting in the useless generation of heat. This makes it necessary to delay slightly the time at which the transistor Q.sub.3 turns on.
Moreover, the prior art involves problems in terms of the current efficiency and circuit arrangement thereof. That is, the control circuit 3 requires the two signal output transistors Q.sub.4 and Q.sub.5 in order to supply the respective transistors Q.sub.2 and Q.sub.3 with the control signals SA and SB.